Method of preparing silicon dioxide



Aug. 31, 1965 Filed Nov. 1, 1961 LOG K G. L. FLEMMERT METHOD OFPREPARING SILICON DIOXIDE SiF 2H2O24HF SiOz F/GI 5 Sheets-Sheet l Aug.31, 1965 G. L. FLEMMERT 3,203,759

METHOD OF PREPARING SILICON DIOXIDE Filed Nov. 1, 1961 3 Sheets-Sheet 2Fla 2 $15, 2nH O24HF SiO n=|0 "=2 90 so 0 CONVERSION Aug. 31, 1965 G.FLEMMERT 3,203,759

METHOD OF PREPARING SILICON DIOXIDE Filed Nov. 1, 1961 3 Sheets-Sheet 3United States Patent 3,293,759 3 F PREPARKNG SiLlCfiN DEQXEEE GostaLcnnart Fiemmert, Humngatan 22, Nyashamn, iveden Filed Nov. 1, 1961, et.149,343 Claims priority, application Sweden, Nov. 3, 1959, 10,533/ 6t? 6Claims. (Ci. 23-182) The present invention relates to a method ofpreparing from gaseous silicon fluorine compounds finely divided silicondioxide having an average particle size of from 2 to 200 mu, preferablyfrom to 50 mg, and little or no conglomeration to form large aggregates.

it is known that silicon fluorine compounds, for example, silicontetrafluoride and volatilized fluosilicic acid, can react in the gaseousphase with water vapor to form finely divided silicon dioxide ofdesirable particle size. The reaction can be carried out in manydifferent ways. Thus, for example, as escribed in US. Patent No.2,819,151, the silicon fluorine compound can be introduced into a flameof burning hydrocarbons or hydrogen gas, whereby silicon dioxide andhydrogen fluoride are obtained together with the exhaust gases formedduring the combustion. Another method of carrying out the reaction, asdescribed in US. Patent No. 2,535,036, is to mix in gaseous phase thesilicon fluorine compound with superheated steam, whereby the fluorinecompound is at least partly hydrolyzed, with the formation of finelydivided silicon dioxide and hydrogen fluoride, according to thefollowing reaction:

where n is the multiplying factor by which the number of moles of waterare multiplied when taken in excess of the stoichiometric molar amount,2.

The thermodynamics of this reaction are such that it is quite ditflcultto carry out on a large scale, thus it presents special problems usuallynot encountered with metal halides other than silicon fluorinecompounds. he equilibrium constant for the above reaction has beencalculated by F. A. Lanfesty, 'l. O. Farr and I. C. Brosheer (ind. Eng.Chem. 44, 1448 (1952)) and I. G. Ryss (J. Phys. Chem. 14, 571 (1940)).According to Laniesty et a1.

where K is the equilibrium constant at atmospheric pressure and T is theabsolute temperature. The variation of the equilibrium constant with thetemperature is shown diagrammatically in FIG. 1. From the above equationfor the equilibrium constant, the degree of conversion for the abovereaction can be calculated according to methods well known in physicalchemistry. Calculations have been carried out for different temperaturesand with u the above reaction being :1, :2 and :10, that is, with thestoichiometrically required amount of water vapor, with 100% excess ofWater and with a -fold excess or" water. The results of thesecalculations are shown diagrammatically in FIG. 2.

As the diagram shows, high temperature and a large excess of water favorthe conversion of silicon tetrafluoride to silicon dioxide. For example,7% or" the silicon tetrailuoride is converted at a temperature of 1000C. if the excess of water is ten times larger than the amount which isrequired stoichiometrically, but the degree of conversion is only 43%when the stoichiometric amount of water is used. At a temperature of 400C., the degree "ice of conversion is when 11:10 and about 5% when 11:1.

The calculations above lead one to expect what actually occurs inpractice, and constitutes an important obstacle to the commercialproduction on a large scale of finely divided silicon dioxide fromsilicon fluorine compounds in the gaseous phase. it the gaseous reactionproduct mixture is cooled before separation of silicon dioxide andhydrogen fluoride, the reaction is reversed, and the finely dividedsilicon dioxide which is formed at high temperatures now reacts with thehydrogen fluoride, regenerating silicon tetraiiuoride according to thereaction This reverse reaction becomes significant at temperatures below600 C., and increases in rate as the temperature drops. Experiments haveshown that the finely divided silicon dioxide, which has a very largesurface area, rapidly reacts with the hydrogen fluoride at temperaturesbelow 0 C. Thus, if the temperature of the reaction mixture is loweredbetween 600 C. by indirect cooling, the silicon dioxide will beconverted to a great extent to silicon tetrafluoride, thus resulting ina low yield and an uneconomical process.

The reaction proceeds in the direction of silicon dioxide formation at660 C. and over, and good yields of silicon dioxide can be obtained, butother difficulties arise. The gas containing hydrogen fluoride is highlycorrosive at such high temperatures, and thus it is diflicult to find asuitable construction material for the separators. In addition, thesilicon dioxide particles are rapidly disproportionated at hightemperatures, so that the larger particles grow larger whereas thesmaller particles grow smaller and eventually disappear, so that aproduct of larger particle size is obtained which is less active andthus less valuable.

The difliculties described above have made it impossible to use silicontetraiiuoride and other silicon fluorine compounds in preparing finelydivided silicon dioxide, though such silicon fluorine compounds areavailable in large quantities as cheap by-products.

in accordance with this invention, silicon dioxide as obtained by thevapor phase reaction of silicon fluorine compounds and water can bemaintained in active form as particles of from 2 to 200 me in size bydiluting the mixture of gas lay-products and suspended silicon dioxidefrom the reaction zone with an inert gas havinr a lower temperature thanthat or" the reaction mixture and in an amount sufiicient to reduce thereaction mixture temperature to below 630 C., and preferably to withinthe range from 350 to 575 C., but above the dew point of the dilutedmixture. The silicon dioxide is then separated from the cooled dilutedsuspension. A good yield of high quality silicon dioxide is obtained, asurprising result in view of the reversibility of the reaction at suchtemperatures.

For cooling the reaction mixture, any inert gas can be used, i.e., anygas which will not react with any of the components desired to berecovered from the reaction mixture. Suitable gases are exemplified byair, Water vapor, nitrogen, carbon dioxide, neon, helium, argon and likerare gases, and exhaust gases from combustion processes. The preferreddiluent is the exhaust or by-product gases from the reaction afterfinely divided silicon dioxide has been separated therefrom, and theyhave been cooled to a suitable temperature, for the reason that nolowering of the concentration of the gaseous components occurs, therebyfacilitating recovery of hydrogen fluoride and unreacted silicontetrafiuoride therefrom, if such be desired.

The quantity of cooling gas admixed and its temperature are so adjustedas to obtain the temperature to which the reaction mixture is to bebrought. These variables can thus be controlled within wide limits,according to the desired degree of cooling. It has been found inpractice that from one to eight times the volume of the reaction mixturegases is the optimum range of operation. The quantity of gas admixed issuitably such that the resulting temperature is below 600 C. but abovethe dew point of the diluted mixture. Within this temperature range itis comparatively easy to overcome the practical problems involved in thesilicon dioxide separation step, such as corrosion andmechanical-strength problems, for example.

The mode of admixture of cooling gas is not critical, and can be carriedout in a variety of ways. Thus, for instance, in using the process andapparatus described in US. Patent No. 2,819,151, for instance, at columnline 67 through column 6 line 3'7, the gas may be introduced behind theflame in a manner to form a curtain of gas surrounding the same. Thiscurtain may be directed either parallel to the direction of flow of theflame, or tangentially thereto, resulting in a vortex action.

It has been found particularly advantageous to introduce the cooling gasat the end of the reaction zone. This results in the advantage that thereaction zone itself will not be cooled or disturbed, whereas thereaction product will be rapidly cooled down after the reaction. Inorder to obtain most effective mixing, the cooling gas may be introducedthrough suitably shaped nozzles.

After admixing the cooling gas, silicon dioxide separation can becarried out by means of a mechanical precipitator. The term mechanicalprecipitator here denotes all types of precipitators except filters andelectrostatic precipitators. As examples of mechanical precipitators,there can be mentioned cyclones and centrifugal precipitators,impingement precipitators and gravimetric precipitation chambers. It hasnot been possible to separate the finely divided silicon dioxideobtained after cooling and dilution in accordance with this invention bymeans of filters or electrostatic precipitators. Teflonpolytetrafluoroethylene or ceramic filters give a recovery of silicondioxide of as low as 2 to 20%, and the process is therefore uneconomicalif these are used. Electrostatic precipitators also give poor recoveryof silicon dioxide and the process again is uneconomical.

The plant comprises a burner 1 arranged in a combustion chamber 2 andhaving supply conduits 3, 4, 5 for combustible gas, silicontetrafluoride and air. The cooling and diluting gas is introduced intothe combustion chamber 2 through a conduit 6. Three series-connectedcyclones 7, 8 and 9 are provided within which finely divided silicondioxide is separated, and discharged through the common conduit 10.Exhaust gases are withdrawn at the top of cyclones 7 and 8 andcon-ducted to the top of the next cyclones, 8 and 9, respectively,through conduits 11 and 12, from the last cyclone 9 the exhaust gasesare transferred to an absorption tower (not shown), through a conduit13. The desired proportion of the exhaust gases as required for coolingis tapped off through a conduit 14, and transferred to a gas cooler 15which in its turn communicates through a conduit 16 with a blower 17serving to supply cooling and diluting gas to the combustion chamberthrough the above-mentioned conduit 6.

IGURE 3 illustrates in diagrammatic form a suitable plant for carryingout the process of the invention. The kg. silicon dioxide of an averageparticle size of 12 m lowing examples.

Example I A continuous flow of 1200 litres/minute of a combustible gasconsisting of 80% of hydrogen, 15% of methane and 5% of ethane wasadmixed with 4240 litres/ minute of air and 330 litres/minute of silicontetrafluoride, and was supplied with 1410 litres/minute of air to theburner of the plant shown in FIGURE 3. The mixture was burnt, wherebythe silicon tetrafluoride was converted into finely divided silicondioxide and hydrogen fluoride. The reaction products from the flame wereadmixed with exhaust gases from the reaction which had been freed fromtheir content of silicon dioxide, cooled to 150 C. in the cooler 15, andreturned to the burner through conduit 6. The amount of cooling gasesadmixed was 3.2 times the quantity of gases formed in the reaction,resulting in a temperature of the mixture of 550 C. as it emerged fromthe burner. The gaseous mixture thus obtained was passed through thethree seriesconnccted cyclones 7, 8 and 9 in which the finely dividedsilicon dioxide Was separated and collected at 10. 34.6 kg. silicondioxide of an average particle size of 12 Til 1. were obtained per hour.This is 69.9% of the theoretical yield. The exhaust gases in excess weretransferred via conduit 11 to an absorption tower, in which the hydrogenfluoride and unrcacted silicon tetrafluoride were recovered.

Example II The procedure of Example I was repeated except that noexhaust gases from the reaction were recycled but instead 26 m. /minuteair having a temperature of C. were introduced through conduit 6. Thetemperature of the resulting gaseous mixture was 560 C. In the cyclones7, 8 and 9 silicon dioxide was precipitated and collected at 10. 33.4kg. silicon dioxide of an average particle size of 14 m were obtained.

Example III The same amounts of gases were fed to the burner as inExample I. Through conduit 6 exhaust gases from the cooler 15 werereturned to the combustion chamber. The amount of cooling gases was 8times the quantity of gases formed in the reaction, and the temperatureof the gases was 100 C. resulting in a temperature of the mixture of 290C. In the cyclones 29.5 kg. silicon dioxide were precipitated per hour.The particle size of the product was 11 m Example IV A continuous flowof 1200 litres/minute of the same combustible gas as mentioned inExample I was admixed with 330 litres/minute of silicon tetrafluoride,2000 litres/ minute of air and 450 litres/minute of oxygen. A mixture of500 litres/minute of air and 200 litres/minute of oxygen was alsosupplied to the burner of the plant shown in FIGURE 3. Through conduit 6cooled reaction gases were returned to the combustion chamber. Thetemperature of the cooling gases was C. and the amount was 2.2 times thequantity of gases formed in the reaction resulting in a temperature ofthe mixture of 600 C. Per hour 35.7 kg. silicon dioxide were separatedin the cyclones and collected at 10. The average particle diameter ofthe product was 187 mg.

I claim:

1. A process for recovering at temperatures below about 600 C.finely-divided silicon dioxide having an average particle diameterwithin the range of 2 to 200 me from the hydrogen fluoride-containinggaseous reaction product obtained by reacting silicon tetrafluoride withwater in the gaseous phase at temperatures above about 600 C. whileinhibiting reverse reaction of the silicon dioxide with hydrogenfluoride to produce silicon tetrafluoride and water, which comprises (1)diluting the reaction product with an inert gas having a lowertemperature than the reaction product in a volume to inhibit saidreverse reaction of silicon dioxide with hy drogen fluoride,simultaneously reduce the temperature of the reaction product to below600 C., and thereby maintain the average silicon dioxide particle sizediameter within the range from 2 to 200 my; and (2) recovering suchsilicon dioxide from the reaction product.

2. A process in accordance with claim 1 in which the silicon dioxide isrecovered from the reaction product by a mechanical dust separator.

3. A process in accordance with claim 1 in Which the inert gas is thecooled mixed gases present in the gaseous reaction product.

4. A process in accordance with claim 1 in which the gaseous reactionproduct is obtained by reacting silicon tetrafluoride with Water in aflame.

5. A process in accordance with claim 1 which comprises diluting thegaseous reaction product with from 1 to 8 volumes of the inert gas.

6. A process in accordance with claim 1 in which the inert gas is at atemperature higher than the dew point but lower than 500 C.

References Cited by the Examiner UNITED STATES PATENTS 2,347,496 4/44Merskat et a1. 23-182 5 2,437,334 3/48 Roetheli 231 X 2,631,083 3/53Engelson et a1. 23182 2,791,490 5/57 Wilcox 23182 2,819,151 1/58Flemmert 23182 3,105,742 10/63 Allen et a1. 23202 10 3,130,008 4/64Stokes et a1. 23--182 FOREIGN PATENTS 1,047,756 12/58 Germany.

20,697 9/08 Great Britain. 15 of 1907 MAURICE A. BRINDISI, PrimaryExaminer.

1. A PROCESS FOR RECOVERING AT TEMPERATURES BELOW ABOUT 600*C.FINELY-DIVIDED SILICON DIOXIDE HAVING AN AVERAGE PARTICLE DIAMETERWITHIN THE RANGE OF 2 TO 200 MU FROM THE HYDROGEN FLUORIDE-CONTAININGGASEOUS REACTION PRODUCT OBTAINED BY REACTING SILICON TETRAFLUORIDE WITHWATER IN THE GASEOUS PHASE AT TEMPERATURES ABOVE ABOUT 600*C. WHILEINHIBITING REVERSE REACTION OF THE SILICON DIOXIDE WITH HYDROGENFLUORIDE TO PRODUCE SILICON TETRAFLUORIDE AND WATER, WHICH COMPRISES (1)DILUTING THE REACTION PRODUCT WITH AN INERT GAS HAVING A LOWERTEMPERATURE THAN THE REACTION PRODUCT IN A VOLUME TO INHIBIT SAIDREVERSE REACTION OF SILICON DIOXIDE WITH HYDROGEN FLUORIDE,SIMULTANEOUSLY REDUCE THE TEMPERATURE OF THE REACTION PRODUCT TO BELOW600*C., AND THEREBY MAINTAIN THE AVERAGE SILICON DIOXIDE PARTICLE SIZEDIAMETER WITHIN THE RANGE FROM 2 TO 200 MU; AND (2) RECOVERING SUCHSILICON DIOXIDE FROM THE REACTION PRODUCT.